John J. Roller, Construction Technology Laboratories, Inc., Robert N. Bruce, Tulane University, and Henry G. Russell, Henry G. Russell, Inc.
The Louisiana Department of Transportation and Development (LA DOTD) built its first high performance concrete bridge—Charenton Canal Bridge* in 1999. Construction of the State’s second high performance concrete bridge is scheduled to commence in the fall of 2002. The new bridge will incorporate 72-in. (1.83-m) deep bulb-tee girders with a specified concrete compressive strength of 10,000 psi (69 MPa) and 0.6-in. (15.2-mm) diameter prestressing strands. To provide assurance that these girders will perform satisfactorily, a research program was initiated to evaluate the structural performance under shear loading conditions. This research is sponsored by the Louisiana Transportation Research Center.
Three 96-ft (29.3-m) long, 72-in. (1.83-m) deep bulb-tee girders were designed and fabricated for the research program. Details incorporated in the test girders were based on prototype bridge designs prepared by the LA DOTD. The prototype bridge used a span length of 95 ft (29.0 m) and a girder spacing of 13 ft 6 in. (4.11 m).The first girder (BT6) was designed based on the AASHTO Standard Specifications for Highway Bridges. Shear reinforcement consisted of individual stirrups at one end and welded wire reinforcement at the opposite end. Design of the other two girders (BT7 and BT8) was based on the AASHTO LRFD Bridge Design Specifications. Two different shear reinforcement designs were developed for these two girders based on two different assumptions about strand development length and the contribution of the longitudinal reinforcement to the shear strength. Girder BT7 contained individual stirrups at both ends. Girder BT8 contained welded wire reinforcement at both ends. In all three girders, design yield strengths for the individual stirrups and welded wire reinforcement were 60 ksi (414 MPa) and 70 ksi (483 MPa), respectively.
The concrete mix used for the girders had specified compressive strengths of 7000 psi (48 MPa) at release of strands and 10,000 psi (69 MPa) at 56 days. After fabrication at a plant in Mississippi, the three girders were shipped by road to Construction Technology Laboratories, Inc. in Skokie, Illinois for testing. Prior to testing, an 8-in. (203-mm) thick, 10-ft (3.05-m) wide reinforced concrete deck was cast on each girder. The high performance concrete used for the deck slabs had a specified 28-day compressive strength of 4200 psi (29 MPa).
Each girder end was tested separately to evaluate static shear strength performance. Load was applied in increments to each girder end at three load points until either the strength of the girder or the safe working capacity of the testing hardware was reached. The measured shear strength of each girder end was compared to the calculated strength based on the design material properties and applicable AASHTO specifications.
As indicated in the bar chart, measured shear strengths consistently exceeded the design strengths. The reported measured strengths for the BT7-L and BT8-L ends were limited by the capacity of the loading hardware and, therefore, are less than the true shear strength. For the four tests where the true shear strength was measured, the test results indicate design provisions of both the AASHTO Standard Specifications and LRFD Bridge Design Specifications provide comparable levels of conservatism in predicting shear strength. The tests also indicate that the use of welded wire reinforcement at a design yield strength of 70 ksi (483 MPa) is an alternative to conventional deformed bars for shear reinforcement.
*See HPC Bridge Views, Issue No. 8, March/April 2000.